Superconductive magnet construction

ABSTRACT

IN SUPERCONDUCTIVE MAGNETS, THE MAGNETIC FIELD WITHIN THE BODY OF THE MAGNET VARIES FROM POINT TO POINT. BOTH THE CURRENT CONDUCTIVITY AND THE HEAT CONDUCTIVITY OF A NORMAL METAL DECREASES AS THE FIELD TO WHICH IT IS SUBJECTED INCREASES, WHEREBY THE NORMAL COATING ON THE RIBBON, HAVING A SUPERCONDUCTIVE CORE, WITH WHICH THE MAGNET IS WOUND PROVIDES LESS SHUNTING ACTION FOR THE SUPERCONDUCTIVE CORE OF THE RIBBON IN THE HIGH FIELD PORTIONS OF THE MAGNET. ALSO, AND FOR THE SAME REASON, THE NORMAL METAL PORTION OF THE INTERLAYER SHEETS WHICH ARE PLACED BETWEEN WINDING LAYERS OF THE MAGNET PROVIDE LESS FLUX MOTION PROTECTION IN THE HIGH FLUX FIELD PORTIONS OF THE MAGNET THAN IN THE LOW FIELD PORTIONS OF THE MAGNET. TO CAUSE THE STABILITY OF THE MAGNET TO BE UNIFORM THROUGHOUT ITS BULK, THE NORMAL METAL COATING ON THE RIBBON AND THE NORMAL METAL SHEET PORTION OF THE INTERLAYER SHEETS ARE MADE THICKER FOR THE HIGHER FIELD PORTIONS OF THE MAGNET THAN FOR THE LOWER FIELD PORTIONS THEREOF.

Jan. 26, 1971 H. c. S CHINDLER 3,559,127

v SUPERCONDUCTIVE MAGNET CONSTRUCTION I Filed May 6, 1968 INVENTOR HEMLY CScumoLEL BY W llll/l/l/IA United States Patent 3,559,127 SUPERCONDUCTIVE MAGNET CONSTRUCTION Henry C. Schindler, East Brunswick, N.J., assignor to RCA Corporation, a corporation of Delaware Filed May 6, 1968, Ser. No. 726,739 Int. Cl. H01f 7/22 US. Cl. 335-216 4 Claims ABSTRACT OF THE DISCLOSURE In superconductive magnets, the magnetic field within the body of the magnet varies from point to point. Both the current conductivity and the heat conductivity of a normal metal decreases as the field to which it is subjected increases, whereby the normal coating on the ribbon, having a superconductive core, with which the magnet is wound provides less shunting action for the superconductive core of the ribbon in the high field portions of the magnet. Also, and for the same reason, the normal metal portion of the interlayer sheets which are placed between winding layers of the magnet provide less flux motion protection in the high flux field portions of the magnet than in the low field portions of the magnet. To cause the stability of the magnet to be uniform throughout its bulk, the normal metal coating on the ribbon and the normal metal sheet portion of the inter layer sheets are made thicker for the higher field portions of the magnet than for the lower field portions thereof.

BACKGROUND This invention relates to superconductive magnet construction. Superconductive magnets may be made by winding a ribbon comprising a superconductive core coated with a layer or normal metal in a helical manner to provide a plurality of serially connected winding layers, the several layers being insulated from each other by three layer interlayer sheets comprising two sheets of an insulator and an intermediate sheet of a normal metal. The purpose of the normal coating portion of the ribbon is to carry heat away from the superconductive portion of the ribbon and also to shunt current carried by the superconductive portion of the ribbon (when it is superconductive) around the superconductive portion when it becomes normal due, for example, to heating arising from magnetic flux motion. The purpose of the normal metal layer portion of the interlayer sheet is to reduce the incidence of flux motion, since flux motion produces heat and may cause the superconductor to become normal, and also to carry heat away from the windings. Therefore, the normal metal portions of the ribbon and of the interlayer sheets tend to prevent the super conductive ribbon or any part thereof from becoming normal and therefore they comprise stabilizing means. Since the magnetic field produced by the completed magnet is greatest at the center of the magnet and decreases in intensity in a direction perpendicular to the axis of the magnet to zero at a point within a magnet and then reverses its direction and increases to .a second lower maximum value, the ribbon of the innermost winding layer and the interlayer sheet between the innermost layers are subjected to the greatest field. The winding layers and the interlayer sheets surrounding the innermost winding layer are subjected toa field which is less for each successive layer or sheet in an outward direction to the zero point and then is subjected to a field which increases. The electrical conductance and the heat conductance of the normal portion of the ribbon and of the sheet both decrease as the magnetic field to which they are subjected increases. Therefore, in the high field portion of the magnet, less stabilization is provided by the same thickness of normal metal coating on a ribbon and the same thickness of the normal metal portion of the interlayer sheet than in the lower field portion of the magnet.

One way to provide substantially uniform stability throughout the bulk of the magnet while using constant thickness of normal metal on the ribbon and in the interlayer sheets is to cause less current to flow through a ribbon which is in thehigh field portion of the magnet than in the low field portion thereof. This has been accomplished by electrically isolating the several layers and by supplying them by different current sources which supply more current to outer winding layers than to inner winding layers. These several sources must be unicontrolled in a manner such that each source at all times provides the same percentage of its maximum current as the other sources during the build up of current in a magnet to its rated value. The use of many current sources and of a unicontrolling means is expensive.

Another known construction of a superconductive magnet is winding it \with ribbon so heavily coated with normal metal that the magnet is fully stabilized when the magnet is operating at its rated flux. Magnets so controlled have so low a form factor, that is, so much of the volume of the magnet is used up by the thick normal layer on the ribbon, that the field produced thereby is reduced to an undesired degree, resulting in a low field magnet. If such a fully stabilized magnet is built, it becomes very bulky, requiring a large Dewar system including a great deal of helium, whereby a fully stabilized superconductive magnet installation is expensive.

SUMMARY In accordance with the invention each of the several layers of the magnet is wound with a superconductive ribbon having an unvarying cross sectional area of normal coating along the length thereof. However, the cross sectional area of the normal coating portion of the ribbon varies from layer to layer in such a manner that the current shunting action and heat conduction action of the normal coating in each layer is the same as that in each other layer, although the magnetic field to which the several layers are subjected is different. To this end, the thickness of the normal coating portions of the ribbons in the several layers is progressively greater in cross section in a direction away from the magnetic field zero point in the magnet. Also as an additional means for stabilizing the magnet, the thickness of the normal conductive portion of the interlayer sheets are increased from sheet to sheet in a direction away from the magnetic field zero point in the magnet, to keep the heat and electrical conductivity of the several sheets equal, although they are subjected to different magnetic fields, thereby a magnet is produced whose stability is substantially uniform in all parts thereof throughout the volume of the magnet. No attempt is made to fully stabilize the magnet so constructed, whereby the magnet so constructed has a relatively high form factor.

BRIEF DESCR IPTION The invention may be better understood upon reading the following description in connection with the accom panying drawing in which FIG. 1 is a partial elevational view of a partially completed superconductive magnet embodying the invention,

FIG. 2 is a cross section taken along the line 2-2 of FIG. 1, and

FIG. 3 is a sectional view of a superconductor of the type that may be used in winding the magnet of FIGS. 1 and 2.

- e DESCRIPTION" Referring first to FIGS.' 1 and 2, a superconductive magnet may be wound. on awinding spool having a flange 12 at each end thereof (only one of the flanges 12 being shown) and a center tube 14. This spool 10 may be of any material which is sufiiciently physically strong as to act as a core for the superconductive magnet to be wound thereon. The spool 10 is usually made of aluminum or stainless steel. As shown in FIG. 2, one or more layers of insulation 16 and 18 are placed on the tube 14. Also, a layer of insulation 20 is provided on the inside surface of the flanges l2.

Shorting bars 22 are laid on the insulation 18, these shorting bars being strips of copper or any other good normal conductor that does not have superconducting properties. The shorting bars are laid on the insulation 18 in such a direction that they each contact the turns of a layer of the winding of a superconductor ribbon or conductor, to be described, in several places. As illustrated, the shorting bars 22 are laid on the insulation 18 in a direction parallel to the axis of the spool 10. The number and cross sectional area of the bars 22 are chosen in a known manner so as to provide an alternate path for the current in a portion of a conductor when that portion of the conductor has gone normal, until that portion of the conductor becomes superconductive again, thereby increasing the stability of the magnet. Yet, the cross sectional area of the bars must not be so great that they increase the time constant, that is the time it takes to build up the current in the superconductive magnet to its rated value, to an excessively long period of time. A superconductive ribbon or conductor 24 is carefully wound in a helical manner from one end of the spool 10' to the other end thereof over the bars 22. The conductor 24 is wound with a uniform tension and in such a manner as to provide a uniform distance between the adjacent edges of the turns. A connection (not shown) is made to one end of the superconductive ribbon 24, this connection extending out of the superconductive magnet beyond the flange 12. The ribbon or conductor 24 may comprise a stainless steel substrate 26 as shown in FIG. 3, a layer of superconductive material 28 such as niobium stannide on the substrate 26 and a layer 30 of normal conductor such as silver or copper on the superconductive layer 28.

When a complete one-conductor thick winding layer has been wound on the bars 22, a composite interlayer sheet 32 is next wound around the completed layer. The interlayer sheet 32 comprises an insulating film 34, a sheet of conductor 36 which remains normal at cryogenic temperatures, such as copper, and another insulating film 34. The interlayer sheet 32 extends for more than 360 whereby the end portions thereof overlap. However, the conducting sheet 36 provides an interrupted, as distinct from a short circuited, turn due to the fact that the overlapping portions of the sheet 36 are insulated from each other by the insulating films 34. The overlap is not shown in FIG. 2. The interlayer sheet 32 also acts in a known manner to make the superconductive magnet more stable, that is to reduce the tendency of the magnet to become normal during buildup of the field therein to its rated value by reducing the motion of the flux in the magnet and by carrying heat away from the superconductive ribbon.

More shunting bars 22 are positioned on the composite interlayer sheet 32 and another layer of superconductive ribbon 38 is wound in a helical manner on the bars 22. The superconductive ribbon 38 comprises a normal metal coating of less cross sectional area on the superconductive material for a purpose to be explained. This is indicated diagrammatically in FIG. 2 by showing the thickness of the normal coating 30 to be progressively less in the winding layers that are subjected to lesser magnetic fields. The cross sectional area of the superconductive core itself may be constant from winding layer to winding layer or the cross sectional area of the superconductive core may also be less for the ribbons comprising the winding layer that is subjected to a lesser magnetic field.

Then, another composite interlayer sheet 41} is laid in an overlapping manner on the winding 38, the thickness of the normal metal sheet 42 portion of the other composite layer 40 being less than the thickness of a normal sheet 36 of the previously applied layer 32. Then, more shunting bars 22 and a third layer of superconductive ribbon 44 is wound in a helical manner on the lastmentioned bars 22, the ribbon 44 comprising a less cross sectional area of normal metal coated on the superconductive core thereof than used in the ribbon 38. A third interlayer sheet 46 is laid in an overlapping manner on the winding layer comprising the ribbon 44, this interlayer sheet 46 comprising a still thinner normal metal conductive sheet 48 than the normal metal sheet 42 of the interlayer sheet 40; and the several steps are repeated until the magnet is completely wound. Further winding layers which are positioned where the magnetic field increases may be formed of ribbon having greater cross sectional area of the normal portion thereof, and further interlayer sheets which are positioned where the magnetic field increases may comprise thicker normal metal sheets. Current and metering connections are made to the superconductive ribbons where necessary in the known manner. Each of the windings comprising the several layers are connected in series whereby in effect a single, continuous winding is provided. While, as is noted above the overlapping of the several interlayer sheets is not shown in FIG. 2, the overlapping of the outside sheet 46 is indicated by the reference number 50 in FIG. 1. Furthermore, no attempt has been made to show the described superconductive magnet or any portion thereof to scale in the drawings.

As has been pointed out above, the electrical conductivity and the heat conductivity of normal metal is reduced in amount by a magnetic field to which the magnet is subjected. As stated above, the innermost layer comprising the conductor 24 is subjected to the maximum magnetic field of the magnet and each successive layers counting from the interlayer outward is subjected to a magnetic field of less intensity to the zero point and then the field increase in the reverse direction for further layers. However, in a superconductive magnet in which all of the layers of the winding are connected in series, the conductors comprising all the windings will carry the same current. Therefore, for equal stabilization of the various winding layers of the magnet, the winding layers which are subjected to higher magnetic fields must be wound with a ribbon having thicker normal coating than the winding layers that are subjected to lesser magnetic field, and the interlayer sheets which are subjected to higher magnetic fields must comprise a normal conductive sheet that is thicker than the normal conductive sheet portions of the interlayer sheets that are subjected to a lesser magnetic field. The completed magnet which is stabilized to the same degree throughout its bulk will produce a higher field than the magnet whose stabilization is not the same throughout. This is due to the fact that the points in a magnet that are least stabilized determines the stability of the magnet as a whole, and any normal metal in the magnet that causes higher local stabilization than the least stabilization is wasted. This wasted normal metal takes up room, reducing the form factor of the magnet. Therefore, in a magnet that is stabilized to the same degree throughout, the magnetic field produced thereby is greater than a magnet that is stabilized to differing degrees at dilferent points thereof. While complete stabilization can be achieved by using thick enough normal metal in the various portions of the magnet, use of suflicient normal metal to provide complete stabilization results in larger more expensive magnets for producing a desired magnetic field than partially stabilized magnets. The optimum degrees of stabilization is determined by trade off between cost and desired field as well as limitations on the energization and on the operation of the partially stabilized magnet.

While only one embodiment of the improved superconductive magnet has been described, modifications thereof will suggest themselves to a person skilled in the art. For example, the superconductive ribbons may be of any cross sectional form other than the ribbon like form disclosed, such as round cross section, and may include a substrate 26 or not as desired, provided that the current and heat carrying capacity of the normal metal coating portion of the superconductive ribbon and of the normal sheet portion of the interlayer sheet is sufiiciently great at the magnetic field to which they are subjected. The several layer windings may have the same or diiferent number of turns. It may be convenient to wind several adjacent layers with conductors of the same cross sectional area of normal material instead of using different conductors having different areas of normal material for each Winding layer. Similarly it may be convenient for adjacent interlayer sheets to include normal metal sheets of the same thick ness. Also, a spool 10 having no flange 12 may be used. Only one layer of insulation 16 or 18 may be necessary on the tube 14. If an insulating spool is used, no insulation on the spool may be necessary. Therefore this description is to be considered as illustrative and not in a limiting sense.

What is claimed is:

1. A superconductive magnet having a plurality of helically wound layers of superconductive ribbon, said ribbon comprising a superconductor coated by a normal metal to contribute stability to the magnet, said layers being coaxially arranged, the several layers of ribbon being subjected in the operation of the magnet to different intensity of magnetic field, the ribbon comprising a layer which is subjected to a lesser magnetic field having a lesser cross sectional area of normal conductor than a ribbon that is subjected to a greater magnetic field, the cross sectional area of the normal conductor in each layer being such as to cause the several layers to contribute substantially the same degrees of stability to the magnet.

2. The invention as expressed in claim 1 in which an interrupted turn comprising an interlayer sheet is inserted between winding layers, said interrupted turn comprising a sheet of normal conductive material, the interlayer sheet contributing stability to said magnet, several interlayer turns being subjected to difierent magnetic fields in the operation of the magnet, the interlayer sheet which is subjected to a lesser magnetic field including normal metal sheet of less thickness than the interlayer sheet which is subjected to a greater magnetic field, the thickness of said normal metal sheet being such as to cause the several layer sheets to contribute substantially the same degree of stability to the magnet.

3. A superconductive magnet having a plurality of wound layers of superconductive ribbon, said layers being coaxially arranged, there being an interrupted turn of an interlayer sheet between Winding layers, said interrupted turn' comprising a sheet of normal conductive material, the interlayer sheet contributing stability to said magnet, several interlayer turns being subjected to different magnet fields in the operation of the magnet, the inter- I layer sheet which is subjected to a lesser magnet field including a normal metal sheet of less thickness than the interlayer sheet which is subjected to a greater magnetic field, the thickness of said normal metal sheet be ing such as to cause the several interlayer sheets to contribute substantially the same degrees of stability to the magnet.

4. In combination, a plurality of wound layers of superconductive wire, said wire comprising a superconductor material coated by a normal metal, said layers being subjected to different magnetic field intensities, that portion of the Wire which is subjected to one intensity having a smaller cross sectional area of said normal metal than that portion of wire which is subjected to a greater intensity.

References Cited UNITED STATES PATENTS 3,129,359 4/1964 Kunzler 335-2l6X 3,283,277 11/1966 Hulm et al 3352l6 3,428,925 2/1969 Bogner et al. 335216 OTHER REFERENCES Laverick et al., Large High Field Super-conducting Magnet System, June 1965, pp. 825-830, The Review of Scientific Instruments.

GEORGE HARRIS, Primary Examiner 

